Pin-tumbler locking mechanisms contain a cylinder plug which rotates within a tightly-fitting cylindrical housing or shell. Channels containing elongated top and bottom pin tumblers extend perpendicularly through the cylinder plug and shell. The pin tumblers slide up and down within the channels to provide for a locked and unlocked position. When the top or bottom pin tumbler spans both the cylinder plug and shell, the pin tumbler is in a position of interference and the cylinder plug is locked and therefore unable to rotate within the shell. When the correct key is inserted into the keyway of the lock, the notches on the key contact the bottom pin tumblers and slide the pin tumblers within the channels so that the entire length of the bottom pin tumbler is positioned within the cylinder plug at its outside diameter. As such, the pin tumblers are in a position of non-interference, and the cylinder plug is unlocked thereby allowing the cylinder plug to rotate within the shell when rotational torque is applied by the key.
Locks can be picked, or opened without a key. FIGS. 1A–1G illustrate one conventional lock picking technique. As shown in FIG. 1, a lock housing or shell A is provided with a rotateable cylinder plug B housed therein. A set of channels C extend through the shell A and cylinder plug B and contain spring-loaded pin tumblers D. In the locking mechanism shown in FIG. 1, the pin tumblers D have two parts which can separate when aligned along the shear line E by the correct key (not shown). In order to pick the lock, a tension wrench F is inserted into the keyway G of the lock, as shown in FIG. 1B, and rotational torque is applied to the cylinder plug B. Since the pins D are in a position of interference with the cylinder plug B and shell A, the cylinder plug B is unable to rotate within the shell A. However, due to imperfections and misalignments in the mechanism, the torque applied by the tension wrench F can cause slight rotation of the cylinder plug B which results in small offsets between the channels C in the cylinder plug B and the shell A. This offsetting of the channels C creates a ledge along the surface of the channels C along the shear line E. A pick H is then inserted into the keyway G and used to slide one of the pin tumblers D up its respective channel C so that the end of the pin tumbler D rests on the ledge created along the shear line E, as shown in FIG. 1C. Continued application of the rotational torque causes the pin tumbler D to remain wedged in this position of non-interference. As shown in FIGS. 1D–1F, the pick H is then used to position each of the other pin tumblers D on the ledge one at a time. As shown in FIG. 1G, once all of the pin tumblers D are positioned on the ledge, the cylinder plug B can rotate within the shell A, thereby allowing the locking mechanism to be unlocked.
An alternative to the pin-tumbler lock is the wafer-tumbler locking mechanism. Wafer-tumbler locks require less strict tolerances between components and, therefore, are advantageous in that they are more economical to manufacture than pin tumbler locks. Wafer tumbler locks have thin wafer-shaped tumblers which slide up and down within slots that span both the cylinder plug and shell. The wafer tumblers are spring loaded so that they extend out of the cylinder plug and into a cavity within the lock shell. In this position of interference, the extended wafer tumblers prevent rotation of the cylinder plug within the shell. The center of each of the wafer tumblers has an opening so that a key can be inserted into the keyway and through the wafer tumblers. The correct key contacts the wafer tumblers and moves the wafer tumblers within the slots so that they are retracted from the cavity within the lock shell and positioned within the cylinder plug. So positioned, the wafer tumblers are in a position of non-interference and rotational torque applied to the cylinder plug causes its rotation within the shell and unlocking of the mechanism. Insertion of an incorrect key into the lock keyway will not result in placement of the wafer tumblers in a position of non-interference.
Since wafer tumbler locks are easier to pick, its resistance to picking can be increased by placing a second locking feature within the lock. One such locking feature that has been used in the past is a spring-loaded sidebar. A sidebar is positioned within its own slot in the cylinder plug, the slot cut perpendicular to the slot within which the wafer-tumblers slide. Positioned within a sidebar slot, a sidebar can contact a wafer tumbler. Two types of sidebar can be used, those that are sprung away from the tumblers and those that are sprung toward the tumblers. There are distinct advantages to using the type that is sprung toward the tumblers. For example, a sidebar that is sprung away from the tumblers can be forced into the tumblers and into a position of non-interference by the application of rotational torque. On the other hand, a sidebar that is sprung toward the tumblers will not seat properly in the tumbler upon the application of rotational torque. When the wafer tumbler is in a position of interference, the wafer tumblers contact with the sidebar prevents the sidebar from withdrawing from the cavity within the shell. So positioned, the sidebar spans the cylinder plug and shell and keeps the cylinder plug from rotating within the shell. When the wafer tumbler is in a position of non-interference, the wafer tumbler contact with the sidebar is changed such that the sidebar is no longer held within the cavity of the shell and therefore does not span the cylinder plug and shell. When the sidebar is so positioned, rotational torque causes the cylinder plug to rotate within the shell.
Although wafer-tumbler locks are more economical to produce and are of smaller size than some other tumbler locking mechanisms, pin-tumbler locks for example, they are typically less resistant to picking than pin-tumbler locks. There is a need for a wafer-tumbler locking mechanism that is more pick-resistant.